The current invention pertains to automatic real time monitoring of true color in waste liquids such as pulp and paper mill effluents. More specifically, in one aspect the present invention relates to the automated real time detection of true color in effluents with an on line in situ absorption spectrophotometer. In another aspect, the current invention also pertains to the use of aluminum chlorohydrates polymer blends for the removal of color in pulp and paper mill effluents.
Wood consists of approximately 45% cellulose, 30% hemi-cellulose, 23% lignins and 2% of a mixture of terpenes, resins and fatty acids. Pulping purifies the cellulose and hemi-cellulose from the other components of wood such as lignins, oils and resins and is integral to paper manufacturing.
Chemical pulping is a preferred method for producing very pure cellulose fibers in paper manufacturing. The most common form of chemical pulping is the Kraft pulping process where materials such as wood chips are heated under pressure with an aqueous solution of sodium hydroxide, sodium carbonate, and sodium sulfide (i.e. pulping liquor) to provide a dark brown pulp. Lignins, which are polymers that bind to hemi-cellulose and provide structural rigidity to wood, are degraded by the heated, pressurized caustic solution, which provides pulp substantially enriched in both cellulose and hemi-cellulose. The Kraft pulping process typically removes about 90 to 95% of the lignin found in naturally occurring wood. The dark brown pulp provided by the Kraft process must be refined by bleaching to remove the remaining lignin to manufacture fine paper.
Further purification of the pulp requires removal of both volatile materials such as terpenes and used pulping liquors. The used pulping liquors, referred to as weak black liquor, typically contain large amounts of organic materials. The weak black liquor is typically evaporated to provide a strong black liquor which contains over 50% solids. The highly concentrated solution of lignin, dissolved organic material and pulping liquor additives is then burned in a liquor recovery furnace. The organic material and the lignins are combusted while the pulping liquor additives may be recovered for reuse.
The concentrated organic material in the strong black liquor causes serious problems when accidentally discharged to a waste treatment facility. Liquor losses negatively affect the waste treatment facility and are environmentally detrimental to the receiving body of water through toxic breakdown effects. Currently, no accurate practical method for automatic in situ real time monitoring of liquor discharge to the waste treatment facility exists. Furthermore, real time methods for treating the liquor based on monitoring also do not presently exist.
Bleaching, typically accomplished with chlorine dioxide and sodium hypochlorite, removes the remaining lignins from the pulp provided by the Kraft process. Bleaching solubilizes lignins, thus imparting a large amount of color to the bleaching solution. The amount of color in a discharged waste stream is generally indicative of the stream""s toxicity. Therefore, one focus of current environmental regulations is to regulate the pulping liquor content of the waste streams discharged into public systems and/or waterways. For example, 40 C.F.R. section 430.03 (i.e. the xe2x80x9cClusterxe2x80x9dRule) is particularly applicable to the pulp and paper industry. The color concentration of a liquid is generally referred to as the xe2x80x9ctrue colorxe2x80x9d of the liquid and is typically measured in platinum cobalt (Pt/Co) units.
The volume of highly colored, degraded lignin and wood sugars within an effluent is substantial when bleaching is performed at an integrated paper mill. Numerous treatment schemes are known to those of skill in the art to reduce the color of the effluent. However they tend to be inaccurate and in most cases do not address the real problem of true color. Existing methods include measuring fluid conductivity; Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC). Experimental testing has revealed a lack of reliable correlation between conductivity, pH, COD, or TOC and color concentration. For example, conductivity is affected by regeneration of plant resin exchange units and the use of salt in the process. Further, the conductivity of bleach plant effluent is low compared to conductivity of strong black liquor due to the concentration of salts during evaporation. COD may be easily misinterpreted for several reasons. First, the various oxidants in the bleaching process severely affect color endpoint COD testing. Second, reproducibility of COD testing in a given environment with color endpoint testing methods is difficult. TOC as an indicator reveals only the entire organic content of an effluent. Because various effluents have significant carbon content, individual effluent impact is difficult to measure quantitatively.
Spectrophotometric techniques are considerably more accurate than the aforementioned methods for measuring the true color of pulp and paper mill effluents. In general, spectrophotometric techniques measure the true color of a filtered sample of the effluent. The true color is affected by a number of factors, but in the pulp and paper industry, true color tends to be most significantly affected by degraded lignin bodies, wood sugars and pulping liquor (i.e. sulfide). Although spectrophotometric techniques are useful, current practices require, significant operator intervention and substantial amounts of time to measure the color of pulp and paper mill effluents. Thus, no real time spectrophotometric technique for automatic in situ monitoring of true color currently exists.
Various methods, well known to those of skill in the art, have been used to reduce true color content of paper and pulp mill effluents. For example, branched or linear epichlorohydrin dimethylamine condensation polymers are known to reduce color, which is indicative of toxic substances, in pulp and paper mill effluents. However, application of these polymers without overfeeding is very difficult. Furthermore, low levels of color are difficult to achieve with these condensation polymers since color frequently resolubilizes over time. A similar problem is observed when polydiallyldimethylammoniumchlorides are used to reduce color content of effluents. Epichlorohydrin condensates in combination with dosages of inorganic salts such as aluminum sulfate or ferrous sulfate in the 300 to 1,000 ppm range also effect color reduction. However, this mixture produces a voluminous amount of sludge and requires the addition of caustic soda to maintain the pH of the effluent at required levels. Currently, no method exists that reduces the color of lignin containing effluents to low levels without producing large amounts of sludge and strongly affecting the pH of the effluent.
In view of the foregoing, it should be apparent that the development of a system capable of detecting the true color of waste effluents in real time is highly desirable. Ideally, the real time color measurement should be coupled with process control to provide efficient real time reduction of true color in effluent streams. New chemistry for reducing color in paper and pulp mill effluents would also be desirable.
The present invention relates generally to automated, real time monitoring of true color of effluents. The described methods and apparatus are easily implemented and can provide instantaneous information that may be used to comply with environmental regulations and provide rapid and efficient control of true color in effluents.
In one aspect, the current invention provides a method for automatic real time monitoring of true colors in a liquid. A sample is automatically withdrawn from the liquid (such as a waste stream) in real time and is then automatically filtered in real time. The true color in the filtered sample is automatically detected in real time and the true color content is automatically quantified in real time. The automatic real time monitoring of true colors in a liquid allows for mechanical or chemical control of the true color of the liquid and/or process control of the liquid""s source, which facilitates control of the true color of the liquid itself. By way of example, such control can be accomplished through automated feed forward or automated feed back processes.
In one embodiment, in-situ absorption spectrophotometry is used to detect the true color in the filtered sample. In a more specific embodiment, the true color is detected by absorption spectroscopy at a wavelength between about 400 nm and about 800 nm. In an even more specific embodiment, the true color is detected by absorption spectroscopy at a wavelength in the range of about 430 to 460, as for example about 455 nm. In some embodiments, the spectrophotometry optics are at least periodically cleaned, as for example by periodically adding a biocide to a sample stream to reduce bio-contamination of the spectrophotometry optics.
In one embodiment, the liquid is pulp mill effluent, bleach plant effluent, paper mill effluent, waste water treatment facility effluent or pre-treatment facility effluent. In another embodiment, the liquid is a receiving body of water at a waste water treatment facility or a pre-treatment facility. In pulp plant applications, the real time control of the true color of the effluent stream may be accomplished by controlling the bleaching operation.
In another aspect, the present invention provides an apparatus that automatically monitors true colors in a liquid in real time. The apparatus has a pump that automatically withdraws a sample from the liquid in real time and a filter that automatically filters the sample in real time. The apparatus also has an absorption spectrophotometer that automatically detects true color in the filtered sample in real time and a controller that automatically quantifies the color of the filtered sample in real time.
In one embodiment, the apparatus also has a acid reservoir, a base reservoir, a controller and a pH measuring device for automatically measuring and adjusting the pH of the sample. In another embodiment, the filter is an automatic self-cleaning filter. In a more specific embodiment, the automatic self-cleaning filter has at least two filter units.